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RFC 1144 - 3.2.3 Compressor processing

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### 3.2.3 Compressor processing

The compressor is called with the IP packet to be processed and the compression state structure for the outgoing serial line. It returns a packet ready for final framing and the link level `type' of that packet.

As the last section noted, the compressor converts every input packet into either a TYPE_IP, UNCOMPRESSED_TCP or COMPRESSED_TCP packet. A

TYPE_IP packet is an unmodified copy/16/ of the input packet and processing it doesn't change the compressor's state in any way.

An UNCOMPRESSED_TCP packet is identical to the input packet except the IP protocol field (byte 9) is changed from `6' (protocol TCP) to a connection number. In addition, the state slot associated with the connection number is updated with a copy of the input packet's IP and TCP headers and the connection number is recorded as the last connection sent on this serial line (for the C compression described below).

A COMPRESSED_TCP packet contains the data, if any, from the original packet but the IP and TCP headers are completely replaced with a new, compressed header. The connection state slot and last connection sent are updated by the input packet exactly as for an UNCOMPRESSED_TCP packet.

The compressor's decision procedure is:

- If the packet is not protocol TCP, send it as TYPE_IP.

- If the packet is an IP fragment (i.e., either the fragment offset field is non-zero or the more fragments bit is set), send it as TYPE_IP./17/

- If any of the TCP control bits SYN, FIN or RST are set or if the ACK bit is clear, consider the packet uncompressible and send it as TYPE_IP./18/

If a packet makes it through the above checks, it will be sent as either UNCOMPRESSED_TCP or COMPRESSED_TCP:

- If no connection state can be found that matches the packet's source and destination IP addresses and TCP ports, some state is reclaimed (which should probably be the least recently used) and an UNCOMPRESSED_TCP packet is sent.

- If a connection state is found, the packet header it contains is checked against the current packet to make sure there were no unexpected changes. (E.g., that all the shaded fields in fig. 3 are the same). The IP protocol, fragment offset, more fragments, SYN, FIN and RST fields were checked above and the source and destination address and ports were checked as part of locating the state. So the remaining fields to check are protocol version, header length, type of service, don't fragment, time-to-live, data offset, IP options (if any) and TCP options (if any). If any of these fields differ between the two headers, an UNCOMPRESSED_TCP packet is sent.

If all the `unchanging' fields match, an attempt is made to compress the current packet:

- If the URG flag is set, the urgent data field is encoded (note that it may be zero) and the U bit is set in the change mask. Unfortunately, if URG is clear, the urgent data field must be checked against the previous packet and, if it changes, an UNCOMPRESSED_TCP packet is sent. (`Urgent data' shouldn't change when URG is clear but [11] doesn't require this.)

- The difference between the current and previous packet's window field is computed and, if non-zero, is encoded and the W bit is set in the change mask.

- The difference between ack fields is computed. If the result is less than zero or greater than 2^16 - 1, an UNCOMPRESSED_TCP packet is sent./19/ Otherwise, if the result is non-zero, it is encoded and the A bit is set in the change mask.

- The difference between sequence number fields is computed. If the result is less than zero or greater than 2^16 - 1, an UNCOMPRESSED_TCP packet is sent./20/ Otherwise, if the result is non-zero, it is encoded and the S bit is set in the change mask.

Once the U, W, A and S changes have been determined, the special-case encodings can be checked:

- If U, S and W are set, the changes match one of the special-case encodings. Send an UNCOMPRESSED_TCP packet.

- If only S is set, check if the change equals the amount of user data in the last packet. I.e., subtract the TCP and IP header lengths from the last packet's total length field and compare the result to the S change. If they're the same, set the change mask to SAWU (the special case for `unidirectional data transfer') and discard the encoded sequence number change (the decompressor can reconstruct it since it knows the last packet's total length and header length).

- If only S and A are set, check if they both changed by the same amount and that amount is the amount of user data in the last packet. If so, set the change mask to SWU (the special case for `echoed interactive' traffic) and discard the encoded changes.

- If nothing changed, check if this packet has no user data (in which case it is probably a duplicate ack or window probe) or if the previous packet contained user data (which means this packet is a retransmission on a connection with no pipelining). In either of these cases, send an UNCOMPRESSED_TCP packet.

Finally, the TCP/IP header on the outgoing packet is replaced with a compressed header:

- The change in the packet ID is computed and, if not one,/21/ the difference is encoded (note that it may be zero or negative) and the I bit is set in the change mask.

- If the PUSH bit is set in the original datagram, the P bit is set in the change mask.

- The TCP and IP headers of the packet are copied to the connection state slot.

- The TCP and IP headers of the packet are discarded and a new header is prepended consisting of (in reverse order):

  - the accumulated, encoded changes.

  - the TCP checksum (if the new header is being constructed `in place', the checksum may have been overwritten and will have to be taken from the header copy in the connection state or saved in a temporary before the original header is discarded).

  - the connection number (if different than the last one sent on this serial line). This also means that the the line's last connection sent must be set to the connection number and the C bit set in the change mask.

  - the change mask.

At this point, the compressed TCP packet is passed to the framer for transmission.

16. | It is not necessary (or desirable) to actually duplicate the input packet for any of the three output types. Note that the compressor cannot increase the size of a datagram. As the code in appendix A shows, the protocol can be implemented so all header modifications are made `in place'.
17. | Only the first fragment contains the TCP header so the fragment offset check is necessary. The first fragment might contain a complete TCP header and, thus, could be compressed. However the check for a complete TCP header adds quite a lot of code and, given the arguments in [6], it seems reasonable to send all IP fragments uncompressed.
18. | The ACK test is redundant since a standard conforming implementation must set ACK in all packets except for the initial SYN packet. However, the test costs nothing and avoids turning a bogus packet into a valid one. SYN packets are not compressed because only half of them contain a valid ACK field and they usually contain a TCP option (the max. segment size) which the following packets don't. Thus the next packet would be sent uncompressed because the TCP header length changed and sending the SYN as UNCOMPRESSED_TCP instead of TYPE_IP would buy nothing. The decision to not compress FIN packets is questionable. Discounting the trick in appendix B.1, there is a free bit in the header that could be used to communicate the FIN flag. However, since connections tend to last for many packets, it seemed unreasonable to dedicate an entire bit to a flag that would only appear once in the lifetime of the connection.
19. | The two tests can be combined into a single test of the most significant 16 bits of the difference being non-zero.
20. | A negative sequence number change probably indicates a retransmission. Since this may be due to the decompressor having dropped a packet, an uncompressed packet is sent to re-sync the decompressor (see sec. 4).
21. | Note that the test here is against one, not zero. The packet ID is typically incremented by one for each packet sent so a change of zero is very unlikely. A change of one is likely: It occurs during any period when the originating system has activity on only one connection.

Next: 3.2.4 Decompressor processing

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RFC 1144 - 3.2.3 Compressor processing